Microporous thermal insulation materials are materials which have a lattice structure in which the average interstitial dimension is less than the mean free path of the molecules of air or other gas in which the material is arranged. This results in a thermal conductivity which is less than the molecular conductivity of air or other gas in which the material is used.
The lattice structure is created within a powder material by using a powder with very fine particles which adhere to each other in a chain-like formation. A suitable powder for providing this structure is a finely divided silica normally referred to as silica aerogel or pyrogenic silica.
A block of microporous thermal insulation material can be manufactured from such powders by applying pressure to the powder to compact the particles closely together so that a bond is created at the point of contact between adjacent particles. However, such a block is weak and brittle. Intimate mixing of a reinforcing fibre such as ceramic fibre with the powder before the application of pressure makes it possible to produce a stronger block, although such a block is still brittle and unable to withstand shocks if it is unsupported. An opacifying material such as powdered rutile, ilmenite or other materials having a high refractive index may be added to provide infra-red opacification.